Nitride semiconductors such as GaN, AlN, and InN have a wide band gap and good material properties, and may therefore be used for high breakdown voltage electronic devices or short-wavelength light emitting devices. Particularly, as to field effect transistors (FET) to be used as high breakdown voltage electronic devices, studies are being made in regard to High Electron Mobility Transistors (HEMT), which may be used for high output/high efficiency amplifiers and high power switching devices.
Incidentally, in a conventional HEMT having a horizontal structure (a structure in which the current flows substantially parallel to the substrate surface), when a sufficient amount of breakdown voltage is to be secured for use in high power/high breakdown voltage switching devices, a long inter-electrode length is to be formed. In this case, the chip size of the device to be formed is increased, and the number of chips that may be manufactured from one wafer is decreased, which leads to an increase in the manufacturing cost, resulting in high cost.
Therefore, in high power/high breakdown voltage switching devices, a field effect transistor having a vertical structure (a structure in which the current flows substantially perpendicular to the substrate surface) is garnering attention, because the chip size may be decreased with such a structure.    Patent document 1: Japanese Laid-Open Patent Publication No. 2002-359256    Patent document 2: Japanese Laid-Open Patent Publication No. 2008-53448    Non-patent document 1: Applied Physics Express 1 (2008) 011105    Non-patent document 2: Applied Physics Express 1 (2008) 021104
For example, a field effect transistor having a vertical structure has a source electrode formed on one side of a substrate and a drain electrode formed on the other side of the substrate. Specifically, a description is given of a field effect transistor having a vertical structure, with reference to FIG. 1.
In the field effect transistor having a vertical structure, on a substrate 611 constituted by n+-SiC or n+-GaN, a n-GaN layer 612, a p-GaN layer 613, and a n-GaN layer 614 are formed. On part of the surface of the n-GaN layer 614, a source electrode 621 is formed. Furthermore, an opening part is formed by etching part of the n-GaN layer 614, the p-GaN layer 613, and the GaN layer 612 from the surface of the n-GaN layer 614. An insulating film 615 is formed so as to cover the surface of the n-GaN layer 614 and the surface of the inside of the opening part. Furthermore, in the opening part, a gate electrode 622 is formed via the insulating film 615. On the back side of the substrate 611, i.e., on the side opposite to the side on which the semiconductor layer is formed, a drain electrode 623 is formed.
In a field effect transistor having the above structure, when a voltage is applied between the source electrode 621 and the drain electrode 623, regardless of the potential of the gate electrode 622, a leakage current passing the p-GaN layer 613 is generated. That is to say, in an area other than the area that is the current path indicated by a dashed-line arrow A, a leakage current flowing through the p-GaN layer 613 indicated by a dashed-line arrow B is generated. When such a leakage current is generated, properties of the field effect transistor are degraded.
Thus, there is demand for a semiconductor device and a manufacturing method of a semiconductor device having a high insulation breakdown voltage, a small chip size, and a small amount of leakage current.